Although Wilms tumor is the most common primary renal malignancy in children, it is exceedingly rare in adults and has an estimated incidence of less than 0.2 cases per million. Little is known about the biology of this tumor in adults and clinicians have had to rely on pediatric treatment protocols. Overall, prognosis is worse in adults, though like in children, unfavorable histology and higher stage at presentation confer a worse prognosis.